1. Field of the Invention
This invention relates to a toning station employed in electrophotographic printers and copiers. More particularly, this invention relates to a bearing cap and tension assembly used to increase the toning shell speed, and to toning stations and electrophotographic printers including such a bearing cap and tension assembly.
2. Brief Description of the Related Art
Throughout this disclosure, the term xe2x80x9celectrophotographic printerxe2x80x9d is to be construed to include both printers and copiers employing electrophotographic means for image production. Electrophotographic printers that utilize a dry ink toner-based developer include a developer station having a rotating toning shell that is used to transport the developer mix to the site where the toner is applied to a photoconductor that carries an electrostatic image. The toner interacts electrostatically with the photoconductor, temporarily bonding to the photoconductor before being transferred to the paper.
In some situations, however, the toner is not applied evenly to the photoconductor, resulting in a density xe2x80x9cspikexe2x80x9d on the leading edge of the paper. In other words, the toner is applied more heavily at the leading edge of the page than it is on the rest of the page, leading to possible image quality problems, especially at higher page-per-minute output speeds. As disclosed in U.S. patent application Ser. No. 60/204,882, incorporated herein by reference, this leading edge density problem may be overcome by increasing the rotational speed of the toning shell beyond the speed at which the toning shell rotates on some currently available printers. Therefore, a need exists in the art for an electrophotographic printer having a toning shell capable of rotating at higher speeds than are attainable with some currently available printers.
Typically, the toning shell is driven by means of a chain driven by a sprocket affixed to a shaft that ultimately is driven through a gear box actuated by the main drive motor of the copier. In certain prior art printers, the toning shell was driven directly by a shaft through a single drive chain, with a sprocket mounted on the shaft and a second sprocket mounted on the toning shell. Given the space constraints of the internal volume of many current printers, it is not feasible to simply affix a larger sprocket to the drive shaft, as such a sprocket large enough to generate the required toning shell speed would not fit within the available space. Similarly, it is not feasible to substitute a smaller drive gear on the toning shell shaft, as such a sprocket would necessarily have a smaller diameter than the shaft on which it would be carried, in order to achieve the necessary speeds.
Accordingly, it is an object of the present invention to provide an electrophotographic printer having a toning shell capable of rotating at higher speeds than currently available printers, without substantially re-designing the interior of the printer. In other words, it is an object of this invention to meet the need for increased toning shell speeds within the design constraints imposed by the physical makeup of some current electrophotographic printers.
Moreover, because toner is incompatible with lubrication oils, the drive chains used in electrophotographic printers are typically oiled lightly during manufacture and are then generally not oiled again during their useful life. As a result, the drive chain exhibits wear over time, typified by a slack in the chain. Therefore, it is preferable to include some form of chain tensioner to maintain appropriate chain tension, despite chain wear. Positioning of the tensioner is, however, difficult, in view of the need for the entire toner assembly to move for purposes of adjustment of the individual components of the toning station relative to each other to accomplish optimal transfer of developer through the toning station. This situation is addressed in U.S. patent application Ser. No. 09/442,303, incorporated herein by reference. When the toner assembly is adjusted, the tensioner must also be adjusted, largely defeating the purpose of the tensioner. Accordingly, there is a need in the art for a chain tensioning assembly that does not require adjustment each time the toning station components are adjusted relative to each other.
Thus it is a further object of this invention to provide an integral toning station bearing and tensioning assembly that provides tension to a drive chain without interfering with the movement of the toning station components that must be moved for necessary adjustments.
The present invention solves these and other shortcomings of the prior art by utilizing a faster blender shaft speed and reducing it with sprocket tooth and chain combinations, allows for additional multiplication of the speed of the toning shell without resorting to overly large or small drive sprockets. Additionally, the invention includes an intermediate bearing cap having an integral tensioning sprocket to maintain appropriate chain tension despite chain wear. Because the integral tensioner moves with the bearing cap, no separate adjustment of the tensioner is necessary after toning station adjustment.
In one embodiment, the invention is a bearing cap assembly, including a bearing cap housing, containing at least one bearing, and a tension sprocket assembly secured to the bearing cap housing. In a preferred embodiment, the tension sprocket assembly includes a tension sprocket mounted on a shaft, the shaft secured to a backplate, where the backplate is secured in a channel in the bearing cap body, such that the backplate may move along the channel to provide tensioning pressure to a chain passing over the tension sprocket, and where the tension assembly is biased against the drive chain.
In another embodiment, the invention is an electrophotographic printer having a high speed toning shell. The toning station includes a toner blender, a toner bucket and a toning shell, where the toner blender driven by a blender drive shaft and the toner bucket driven by a bucket drive shaft. A first intermediate drive sprocket is affixed to the blender drive shaft such that rotation of the blender drive shaft directly causes rotation of the first intermediate drive sprocket. A second intermediate drive sprocket is affixed to the bucket drive shaft to permit free rotation of the second intermediate drive sprocket relative to the bucket drive shaft, and an intermediate drive chain connects the first and second intermediate drive sprockets, wherein the second intermediate drive sprocket has a larger diameter than the first intermediate drive sprocket. A first primary drive sprocket is affixed to the second intermediate drive sprocket, such that rotation of the second intermediate drive sprocket directly causes rotation of the first primary drive sprocket, and a second primary drive sprocket is affixed to the toning shell. A primary drive chain connects the first and second primary drive sprockets, wherein the first primary drive sprocket has a smaller diameter than the second primary drive sprocket.
In a preferred embodiment, the electrophotographic printer further includes a tension sprocket assembly to maintain tension on the intermediate drive chain. In one embodiment, the tension sprocket assembly includes a tension sprocket that is biased against the intermediate drive chain to exert tensioning pressure on the intermediate drive chain. The tension sprocket is mounted on a shaft having an axis, the shaft secured to a backplate, and the backplate is secured in a channel in the bearing cap body, such that the backplate may move along the channel to provide tension to a chain passing over the tension sprocket. A spring biases the tension assembly against the drive chain.
In another embodiment, the invention is an electrophotographic developing station having a high speed toning shell, including a toner blender, a toner bucket and a toning shell, where the toner blender is driven by a blender drive shaft, and the toner bucket is driven by a bucket drive shaft. A first intermediate drive sprocket is affixed to the blender drive shaft such that rotation of the blender drive shaft directly causes rotation of the first intermediate drive sprocket. A second intermediate drive sprocket is affixed to the bucket drive shaft to permit free rotation of the second intermediate drive sprocket relative to the bucket drive shaft, and an intermediate drive chain connects the first and second intermediate drive sprockets, wherein the second intermediate drive sprocket has a larger diameter than the first intermediate drive sprocket. A first primary drive sprocket is affixed to the second intermediate drive sprocket, such that rotation of the second intermediate drive sprocket directly causes rotation of the first primary drive sprocket. A second primary drive sprocket affixed to the toning shell, and a primary drive chain connects the first and second primary drive sprockets. The first primary drive sprocket has a smaller diameter than the second primary drive sprocket.